Conventionally, polyimide resin having various superior properties, such as heat resistance and electrical isolation performance, has been widely used in the field of electronics. For example, films made from polyimide resin have been used for a flexible printed board, a TAB tape, and a base film for high-density storage medium. Polyimide resin has been used not only in the form of a film but also in various forms including a sheet and a coating agent. In a case where polyimide resin is used in the form of a film, the film is not always used alone. The film has been widely used in a laminated structure, like a structure in which a copper foil is bonded to the surface of the film with an adhesive, a structure in which the surface of the film is subjected to copper sputtering or copper electroplating, and a structure in which polyimide resin is cast or coated on a copper foil.
With the recent advance of technology of electronic materials and devices, a polyimide film used has been required to have more complex and much more properties other than basic properties such as heat resistance, isolation performance, and solvent resistance. For example, with the downsizing of electrical and electronic devices, flexible printed boards used in the devices are required to have fine wiring patterns. This requires polyimide films that exhibit smaller changes in dimension caused by heating and tension. The lower the linear expansion coefficient is, the smaller the amount of change in dimension caused by heating is. Moreover, the higher the elasticity modulus is, the smaller the amount of change in dimension caused by tension is. However, the polyimide films with high elasticity modulus and low linear expansion coefficient are generally produced using rigid monomers with high linearity, including pyromellitic dianhydride and p-phenylene diamine, for example. This gives rise to the following at least two problems: One problem is that the resulting films are inferior in flexibility and also inferior in bending properties which are required for flexible printed boards. The second problem is that inappropriate use of the monomers produces films having high water absorbency and high moisture expansion coefficient.
For example, in a case where the polyimide film is used for a semiconductor package, the polyimide film is required to have dimensional stability against heat and tension and dimensional stability under the conditions where moisture is absorbed. Therefore, a polyimide film having low linear expansion coefficient, high elasticity modulus, and low moisture expansion coefficient is desirable.
Furthermore, a finer wiring pattern has been formed on a polyimide film for use in a flexible printed board. As a result of this, the demand for metal-laminated polyimide film in which a thin-film metal that allows for the formation of a fine pattern is laminated has been increasing. This demand cannot be met by the method conventionally used in most cases. That is, the method in which a thin copper foil is laminated on the surface of polyimide film with the use of an adhesive such as a thermoplastic polyimide adhesive or epoxy adhesive makes it difficult to laminate a thin copper film suitable for a fine pattern thereon.
In view of this, as a method for producing a metal-laminated board without using an adhesive, a method of directly forming a metal without using an adhesive has been adopted in most cases, like a method in which a thin metal film is formed on the surface of polyimide film by using a sputtering apparatus or a metal vapor deposition apparatus, and then copper is laminated on the metal film by plating. The adoption of this method makes it possible to change a thickness of the metal layer to a thickness in the range from not less than 1 μm to several tens of micrometers as appropriate. In particular, since this technique allows for the formation of a thin film, it also has the feature that a metal layer having a thickness suitable for a fine pattern can be formed.
However, the above method has the following problem: Adherability between the film and the metal layer in the laminated board is lower than adherability obtained by a method using an adhesive, and the adherability obtained by the above method tends to decrease especially when an environmental resistance test is conducted. On this account, the improvement to a polyimide film has been required.
Patent Documents 1 and 2 disclose a polyimide film which is produced by using p-phenylenebis(trimellitic acid monoester anhydride) with the aim of decreasing water absorbency and moisture expansion coefficient. However, the polyimide film disclosed in Patent Documents 1 and 2 is instable in an environmental test, and reliability of the polyimide film decreases when the polyimide film is used for COF or the like.
Further, in the case of a method in which a metal layer is directly formed by sputtering, vapor deposition, or the like without using an adhesive, it is different in its production process from a method in which a metal foil is laminated with the use of an adhesive. For example, metal sputtering requires that a state within the system is changed to a nearly vacuum state in the process of sputtering. However, it is desired that a state within the system should be changed to a vacuum state as soon as possible in consideration of productivity.
Patent Document 3 discloses a polyimide film which is produced by using the following five components: p-phenylenebis(trimellitic acid monoester anhydride); pyromellitic dianhydride; biphenyltetracarboxylic dianhydride; p-phenylene diamine; and diaminodiphenylether. Patent Document 3 also discloses that adherability increases in a case where a metal layer is directly formed on the polyimide film by vapor deposition, sputtering, or the like without using an adhesive. However, the technical feature of Patent Document 3 is that types and compositions of monomers to be used are selected, and therefore totally different from that of the present invention.
Meanwhile, there is a known method in which polymerization is carried out at multiple stages to produce polyimide resin having a block component. For example, Patent Documents 4 and 5 disclose the following method as a method of polymerizing a block component in advance: a polyamic acid consisting of phenylenediamine and pyromellitic dianhydride or a polyamic acid consisting of phenylenediamine and 3,3′-, 4,4′-benzophenonetetracarboxylic acid is polymerized to form a block component of the polyamic acid, and imide is added to the obtained block component, whereby a copolymerized polyimide having a block component is produced. However, neither Patent Document 4 nor Patent Document 5 includes a step of forming a thermoplastic block component.
Thus, the technical idea has never been known of designing a film so that a thermoplastic polyimide block component is present in the film by using non-thermoplastic polyimide resin containing a thermoplastic polyimide block component, and the film is non-thermoplastic as a whole.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-54862
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-72781
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-137486
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-80178
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-119521